TRISTIMULUS COLORIMETERS The first type of colorimeter is the tristimulus colormeter, which uses four color filters to approximate color matching functions. The characteristics of a light source in the tristimulsus instrument must be corrolated to the filters of that specific instrument. Therefore, a tristimulus instrument can have only one light source per set of filters. A tristimuls instrument measures color at three points on the spectral curve. The Hunter D25-9 and the Gardner XL-805 instruments are both tristimulus colorimeters.
SPECTROPHOTOMETRIC COLORIMETERS The second type of colorimeter is the spectrophotmetric colorimeter. These instruments measure spectrophotmetric reflectance and integrates it with the color measurement. These colorimeters do not use filters, but rather seventeen photodiode detectors to measure sixteen points on the spectral curve. These instruments are capable of evaluating color using more than one illuminant. The Macbeth is a spectrophotometric type instrument.
by Michele D. Burkenbine, from background section in a feasibility study of colorimeters, 1981.